InTech-Wind Turbine Gearbox Technologies

8/2/2019 InTech-Wind Turbine Gearbox Technologies

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Wind Turbine Gearbox Technologies

Adam M. Ragheb1 and Magdi Ragheb21Department of Aerospace Engineering

2Department of Nuclear, Plasma and Radiological Engineering,University of Illinois at Urbana-Champaign, 216 Talbot Laboratory

USA

1. Introduction

The reliability issues associated with transmission or gearbox-equipped wind turbines and

the existing solutions of using direct-drive (gearless) and torque splitting transmissions in

wind turbines designs, are discussed. Accordingly, a range of applicability of the different

design gearbox design options as a function of the rated power of a wind turbine is

identified. As the rated power increases, it appears that the torque splitting and gearless

design options become the favored options, compared with the conventional, Continuously

Variable Transmission (CVT), and Magnetic Bearing transmissions which would continue

being as viable options for the lower power rated wind turbines range.

The history of gearbox problems and their relevant statistics are reviewed, as well as the

equations relating the gearing ratios, the number of generator poles, and the high speed and

low speed shafts rotational speeds.

Aside from direct-drive systems, the topics of torque splitting, magnetic bearings and their

gas and wind turbine applications, and Continuously Variable Transmissions (CVTs), are

discussed.

Operational experience reveals that the gearboxes of modern electrical utility wind turbines

at the MegaWatt (MW) level of rated power are their weakest-link-in-the-chain component.

Small wind turbines at the kW level of rated power do not need the use of gearboxes since

their rotors rotate at a speed that is significantly larger than the utility level turbines and can

be directly coupled to their electrical generators.

Wind gusts and turbulence lead to misalignment of the drive train and a gradual failure of

the gear components. This failure interval creates a significant increase in the capital andoperating costs and downtime of a turbine, while greatly reducing its profitability and

reliability. Existing gearboxes are a spinoff from marine technology used in shipbuilding

and locomotive technology. The gearboxes are massive components as shown in Fig. 1.

The typical design lifetime of a utility wind turbine is 20 years, but the gearboxes, which

convert the rotor blades rotational speed of between 5 and 22 revolutions per minute (rpm)

to the generator-required rotational speed of around 1,000 to 1,600 rpm, are observed to

commonly fail within an operational period of 5 years, and require replacement. That 20

year lifetime goal is itself a reduction from the earlier 30 year lifetime design goal (Ragheb &

Ragheb, 2010).


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Fundamental and Advanced Topics in Wind Power190

2. Gearbox issues background

The insurance companies have displayed scrutiny in insuring wind power generation. Theinsurers joined the rapidly-growing market in the 1990s before the durability and long term

maintenance requirements of wind turbines were fully identified. To meet the demand, anumber of units were placed into service with limited operational testing of prototypes.During the period of quick introduction rate, failures during wind turbines operation werecommon. These included rotor blades shedding fragments, short circuits, cracked foundations,and gearbox failure. Before a set of internationally recognized wind turbine gearbox designstandards was created, a significant underestimation of the operational loads and inherentgearbox design deficiencies resulted in unreliable wind turbine gearboxes.The lack of full accounting of the critical design loads, the non-linearity or unpredictabilityof the transfer of loads between the drive train and its mounting fixture, and themismatched reliability of individual gearbox components are all factors that were identified

Fig. 1. Top view of a Liberty Quantum Drive 2.5 MW rated power wind turbine gearbox(Source: Clipper Windpower).


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Wind Turbine Gearbox Technologies 191

by the National Renewable Energy Laboratory (NREL) as contributing to the reducedoperating life of gearboxes (Musial et al., 2007).In 2006, the German Allianz reportedly received 1,000 wind turbine damage claims. Anoperator had to expect damage to his facility at a 4-5 years interval, excluding malfunctions

and uninsured breakdowns.As a result of these earlier failures, insurers adopted provisions that require the inclusion bythe operator of maintenance requirements into their insurance contracts. One of thecommon maintenance requirements is to replace the gearbox every 5 years over the 20-yeardesign lifetime of the wind turbine. This is a costly task, since the replacement of a gearboxaccounts for about 10 percent of the construction and installation cost of the wind turbine,and will negatively affect the estimated income from a wind turbine (Kaiser &Frhlingsdorf, 2007). Figure 1 depicts the size of the Quantum Drive gearbox of a Liberty2.5 MW wind turbine (Clipper Windpower, 2010)The failure of wind turbine gearboxes may be traced to the random gusting nature of thewind. Even the smallest gust of wind will create an uneven loading on the rotor blades,which will generate a torque on the rotor shaft that will unevenly load the bearings andmisalign the teeth of the gears. This misalignment of the gears results in uneven wear onthe teeth, which in turn will facilitate further misalignment, which will cause more unevenwear, and so on in a positive feedback way.The machine chassis will move, which will misalign the gearbox with the generator shaftand may eventually cause a failure in the high speed rear gearing portion of the gearbox.Further compounding the problem of uneven rotor blade loading is the gust slicing effect,which refers to multiple blades repeatedly traveling through a localized gust (Burton et al.,2004). If a gust of wind were to require 12 seconds to travel through the swept area of awind turbine rotor operating at 15 rpm, each of the three blades would be subject to the gust

three times, resulting in the gearbox being subjected to a total of nine uneven loadings in arapid succession.The majority of gearboxes at the 1.5 MW rated power range of wind turbines use a one- ortwo-stage planetary gearing system, sometimes referred to as an epicyclic gearing system.In this arrangement, multiple outer gears, planets, revolve around a single center gear, thesun. In order to achieve a change in the rpm, an outer ring or annulus is required.

Fig. 2. Planetary gearing system.

Sun, generator shaft Annulus, rotor shaft

Planet


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As it would relate to a wind turbine, the annulus in Fig. 2 would be connected to the rotorhub, while the sun gear would be connected to the generator. In practice however, moderngearboxes are much more complicated than that of Fig. 2, and Fig. 3 depicts two differentGeneral Electric (GE) wind turbine gearboxes.

Fig. 3. GE 1P 2.3 one-stage planetary and two-stage parallel shaft (top) and 2P 2.9 two-stageplanetary and one-stage parallel shaft (bottom) wind turbine gearboxes (Image: GE).


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Planetary gearing systems exhibit higher power densities than parallel axis gears, and are ableto offer a multitude of gearing options, and a large change in rpm within a small volume. Thedisadvantages of planetary gearing systems include the need for highly-complex designs, thegeneral inaccessibility of vital components, and high loads on the shaft bearings. It is the last

of these three that has proven the most troublesome in wind turbine applications.In order to calculate the reduction potential of a planetary gear system, the first step is todetermine the number of teeth, N, that each of the three component gears has. These valueswill be referred to as:

, ,sun annulus planetN N and N

as they relate to the number of teeth on the sun, annulus, and planet gears, respectively.Using the relationship that the number of teeth is directly proportional to the diameter of agear, the three values should satisfy Eqn. 1, which shows that the sun and annulus gearswill fit within the annulus.

2sun planet annulusN N N (1)

With Eqn. 1 satisfied, the equation of motion for the three gears is,

2 2 1 0sun sun sunannulus sun planetplanet planet planet

N N N

N N N

(2)

where: sun, annulus, and planet are the angular velocities of the respective gears.Since the angular velocity and is directly proportional to the revolutions per minute (rpm),Eqn. 2 may be modified to Eqn. 3 below.

2 2 1 0sun sun sunannulus sun planetplanet planet planet

N N Nrpm rpm rpm

N N N

(3)

Known values may be substituted into Eqn. 3 in order to determine the relative rpm valuesof the sun and annulus gears, noting the two equalities of Eqns. 4 and 5 below (Ragheb &Ragheb, 2010).

sunsun planet

planet

Nrpm rpm

N

(4)

planetplanet annulus

annulus

Nrpm rpm

N

(5)

Historically, the gearbox has been the weakest link in a modern, utility scale wind turbine.

Following the current trend of larger wind turbines for offshore applications with theirlarger rotor diameters and heavier rotor blades, gearboxes are being subject to significantly

increased loads.Minor improvements in the gearbox lubrication and oil filtration system have increased the

reliability of wind turbines, but to significantly improve the gearbox reliability, the design


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must be changed from the current planetary gear design. This improved reliability isespecially important for offshore applications, as the wind turbines are generally much

larger and the cost of maintenance is much greater.

3. Gearless / direct-drive wind turbines

The Enercon Company of Germany and ScanWind of Norway have served as the pioneersof the Gearless, or Direct-Drive, wind turbine generator. By increasing the number ofmagnetic pole pairs in a generator from the 4 or 6 of conventional generators to 100 or more,the need for a gearbox may be eliminated.In order to produce the 60 or 50 Hz electrical power for the United States or Europe, a 4-polegenerator would have to operate at 1,800 or 1,500 rpm respectively. Increasing the number ofpoles to 6 would decrease the generator rpm to 1,200 and 1,000, respectively. The relationshipbetween the generator rpm, the number of poles n, and the frequency f is given by:

[ ]120rpmf n Hz (6)

A four-pole, 1,800 rpm generator has a frequency:

1,8004 60[ ]

120f Hz

The generators rpm can be expressed as:

( )120generator

f Hzrpm

n (6)

Increasing the number of poles n in a generator to 160 to produce 60 Hz electricity allows itto rotate at a smaller rpm value of:

60120 45

160generatorrpm

The gearing ratio, G, is defined in Eqn. 7.

generator

rotor blade

rpmG

rpm (7)

Using the example of producing electricity at a frequency of 60 Hz with a 4-pole generatorand a rotor blade operating at 15 rpm, Eqn. 7 shows that a gearing ratio G of :

1,800120

15G

between the rotor blade shaft and the generator shaft would be required.If however, a 160 pole generator is used, the gearing ratio drops to:

453

15G


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with all other values held constant.Finally, a 400-pole generator operating on a rotor blade at 18 rpm would yield a gearingratio of:

60120 18400

181

18

generatorrpm

G

A gearing ratio of unity implies that a gearbox would not be needed.The first entrant with a direct-drive wind turbine is widely cited as being Enercon GmbH ofAurich, Germany. They suggest that their annular generator, in addition to precluding theneed for a gearbox, contains a smaller number of moving parts, further contributing theincreased reliability and reduction in frictional losses. Because the operational speeds of thegenerator are much lower, the generator is subjected to little, if any, wear, allowing to

achieve a longer operational life and to handle larger loads.Unique to Enercon, is their manual winding of the copper wire in the stator portion of theannular generator, justified by their use of a continuous wire strand in each generator,which reduces resistive losses. Enercon is very proud of their closed varnish-insulatedwires, rated to Temperature Tolerance Class F (155 F), suggesting that breaks in theinsulation, especially at joint locations, may have been a significant problem in earlygenerator construction. In order to maintain high levels of quality control, Enerconmanufactures its annular generators in the companys own production facilities.Beginning with their first direct-drive wind turbine in 1993, Enercon has dominated thedirect-drive wind turbine market, and in 2007 was fourth in terms of worldwide windturbine market share, capturing 14 percent of the market behind Vestas, GE, and Gamesa.

While in the past a number of competitors in the direct-drive market have filed forbankruptcy or been bought and sold repeatedly, as was the case for Lagerwey being sold toZephyros which was sold to Harakosan, and then ended up in the hands of STX heavyIndustries of Korea, the marketplace appears to have settled with the entrance of industrygiants GE and Siemens. Figures 4 and 5 show annular generators under assembly at theEnercons company manufacturing facilities.One requirement for a direct-drive wind turbine is to not have direct coupling with theelectrical grid. This is due to the fact that wind turbine rotor blades operate within an rpmrange, and with a direct-coupled generator, the output voltage and frequency vary slightly.A DC link and inverter convert the produced energy to parameters suitable for transmission

to the electrical grid. Prior to the development of these active electronic systems, windturbines used capacitors and static Volt-Ampre-Reactive (VAR) systems that were far fromoptimal.Another consideration of direct-drive wind turbines is their increased manufacturing andmaterial costs. When the German company Siemens embarked on a two-year testingprogram for its 3.6 MW direct-drive turbine, Henrik Stiesdal commented that direct-drivewind turbines may become competitive with their geared counterparts near the upper endof turbine sizes (at that time in the 4-6 MW range), and with the test rigs they determined atwhat level direct-drive could be made competitive (Ragheb & Ragheb, 2010). Three yearslater, these comments turned out to be almost prophetic, as the majority of the wind turbinedesigns with rated powers around 1.5 MW still utilize a planetary gear system, while the


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Fig. 4. Multipole annular generators under assembly (Photo: Enercon).

Fig. 5. Rotor and stator of Enercon E-70 wind turbine (Photo: Enercon).


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more recent and larger designs, many of which are marketed for offshore applications, aredesigned around direct-drive generators. It appears that this changeover point betweengeared and direct-drive wind turbines lies in the 1.5-3 MW rated power range.According to the United States Department of Energy, direct-drive generators require large

diameters, which necessitates the use of large amounts of rare earth elements magnets, andconsequently are expensive and require a larger and heavier drivetrain. In addition to this,a small air gap on the millimeter scale is required to be maintained between the rotor andstator to yield sufficiently high flux densities. As a consequence, the tight manufacturingtolerances required and the detailed design to handle the complex loads encountered addanother set of challenges that may set an upper limit on the size of such generators(Department of Energy, 2010).While it was the first, Enercon is not the only company marketing direct-drive windturbines. Japan Steel Works (JSW) has licensed Enercons technology, and competes againstthe likes of Vensys, Leitwind, MTorres, and ScanWind, now acquired by General Electric(GE). Table 1 presents a summary of some of the utility scale direct-drive wind turbines

designs currently available.In late 2009, GE acquired ScanWind for approximately 15 million Euros in what appeared tobe a technology-driven move to reenter the offshore wind turbine sector. GEs previous forayinto offshore wind turbines was the Arklow Bank Wind Park (Ireland) project, in which seven3.6 MW technology demonstrator wind turbines were installed. In contrast to the ScanWinddirect-drive turbines, these wind turbines utilized a three step planetary gear system.The direct-drive approach to the gearbox problem appears to be taking hold quite well onthe largest capacity wind turbines. Due to its lower amount of moving parts, it seems ideallysuited for large offshore applications.Direct-drive may not however solve the existing gearbox problems for all wind turbines

over the rated power range between 1.5 and 10 MW, as it brings with it weight increases ofaround 25 percent and a cost increase of around 30 percent.A further development of the direct-drive solution couples a direct-drive concept withsuperconducting materials, with potential benefits being reduced mass and volume, andconsequently smaller transportation costs and lower loadings on the tower. Cost advantagesof superconducting direct-drive generators will most likely not exist for turbines below a ratedpower of 5 MW, but with turbines of 10 MW already being constructed for offshore use, thisdrivetrain concept may soon become a reality (Department of Energy, 2010).

4. Torque splitting

A different attempt at solving the gearbox problem on 2+ MW sized wind turbines wasundertaken by Clipper Windpower of Carpineria, California. Under a partnership with theUnited States Department of Energy (DOE) and the National Renewable Energy Laboratory(NREL), Clipper developed their 2.5 MW Liberty Wind Turbine, the largest manufactured inthe United States. These turbines, manufactured in 2006, were put into service in early 2007as part of the Steel Winds Project, a superfund location along Lake Erie in Lackawana, NewYork, and the company received a DOE Outstanding Research and DevelopmentPartnership Award in 2007. After only a few months of service, problems were observed intheir distributed gearing-style gearboxes. A subsequent analysis traced the fault toimproper drivetrain timing caused by incorrect gear tolerances on parts arriving fromsuppliers (Robb, 2008).


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Company CountryTurbine RatedPower (MW)

Rotor bladeDiameter (m)

Clipper [Windpower] Marinedeployment date: 2012

UK 10 144

Sway Norway 10 145

Enercon E-126 Germany 7.5 127

Nordex N150/6000 Germany 6.0 150

Xingtan Electric ManufacturingCorporation

China 5.0 -

GE 4.1-113 United States 4.1 113

GE 4.0-110 United States 4.0 110

ScanWind 3.5 (evolved into GE 4.Xseries)

Norway 3.5 90


Leitwind LTW101 Italy 3.0 101

Siemens SWT 3.0-101 Germany 3.0 101

Enercon E-82 Germany 2.0 - 3.0 83

Guangxi Yinhe Avantis Wind Power(in testing)

China 2.5 -

Vensys 2.5 MW Germany 2.5 90 - 100


Leitwind LTW70 Italy 1.7 - 2.0 70.1

Leitwind LTW80 Italy 1.5 - 1.8 80.3

MTorres TWT 1.65/70 Spain 1.65 70



Leitwind LTW77 Italy 1.5 76.6

Leitwind LTW86 Italy 1.5 86.3

Vensys 1.5 MW Germany 1.5 70 - 82

Table 1. Direct-drive wind turbine rated power (MW) and rotor blade diameters.


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Fig. 6. Torque splitting between four electrical generators on the 2.5 MW Clipper Liberty(Image: Clipper Windpower).

Using its patented Quantum Drive Distributed Generation Powertrain, the 2.5 MW Libertywind turbine uses a multiple-path gearbox design to split the torque from its 89 99 meterrotor blades evenly between four generators that are operated in parallel. In contrast to aplanetary gearing system, Clipper utilizes external double helical gears in order to allow forwide faces with their lower deflection sensitivities, smaller diameters, and reducedmanufacturing costs due to lower required tolerances. The gear set for each of thegenerators is designed in cartridge form so as to allow for replacement without requiringthe removal of the gearbox. Additionally, if a fault were to develop in one of the generatorsor cartridged gear sets, the production capacity of the wind turbine is reduced by only 25percent until the problem can be corrected (Mikhail & Hahlbeck, 2006).After selling 370 turbines in 2006, and 825 in 2007, the company appeared to have recovered

from their early quality control problems. Clipper Wind was acquired in December 2010 byUnited Technologies Corporation. On March 24, 2011, Clipper Wind dedicated the firstlarge-scale wind farm on the island of Oahu, which consists of 12 2.5 MW wind turbinescoupled to a 15 MW batter storage system to smooth power output fluctuations. Thisproject was developed by the Boston-based First Wind, one of Clipper Windpowers longstanding customers. As of early 2011, a total of 375 Clipper Windpower turbines arefeatured in 17 projects across the US, with a cumulative rated power of 938 MW.Torque splitting appears to be a cheaper alternative to the direct-drive solution, although itappears that the upper viable limit of torque splitting may lie below that of direct-drivemachines.


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In addition to Clipper Windpower, CWind of Ontario, Canada is introducing a 2 MW, 8-generator wind turbine design. They were testing a 65 kW wind turbine, and haveannounced plans to develop a 7.5 MW turbine. Their design concept may be a hybridbetween torque splitting and a Continuously Variable Transmission (CVT), as they allude to

a friction drive system to absorb sudden wind spikes. A frictional contact drive is one ofthe many types of CVTs. Finally, it should be noted that as shown in Table 1, the subsidiaryof Clipper Windpower, Clipper Marine, has opted for a direct-drive system on its 10 MWturbine. This may provide clues as to the maximum economical size for a wind turbine builtaround a torque splitting concept.

5. Magnetic bearings

A very promising potential solution to the shaft misalignment problem may come from theaerospace and centrifuge uranium enrichment industries in the form of magnetic bearingsor Active Magnetic Bearings (AMBs).

Recent research by NASA, MTU and others point to research in the area of high temperaturemagnetic bearings for use in gas turbine engines to propel aircraft. What appears to be the nextlarge leap in terms of powering commercial transport aircraft is the Geared Turbofan (GTF)engine, which is slated to power the Mitsubishi MRJ, Bombarider C-Series, and A320neo, andmay serve as the platform on which AMBs may be used in aerospace applications. An AMBsystem consists of a magnetic shaft, a controller, multiple electromagnetic coils attached to astator shaft location as shown in Fig. 7. In the event of a failure of the control system, AMBstypically have a passive backup bearing system, which defaults to a rolling element bearing forthe limp home operational mode sensors (Clark et al., 2004).

Fig. 7. Schematic of an Active Magnetic Bearing (Clark et al., 2004).


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The GTF engine is by no means a new concept, as engine maker Pratt and Whitneyunderstood the theoretical justification behind the concept in the early 1980s. The level oftechnology and materials development necessary to meet the stringent safety, reliability, andruggedness requirements of modern gas turbine engines has been achieved lately. The Pratt

and Whitney company suggests that through thousands of hours of development, advances inbearing, gear system, and lubrication design have been made and incorporated into their newfamily of GTFs, with initial reports suggesting promising heat and efficiency data.SAE International reports that Pratt and Whitney uses a self-centering bearing technologythat has all but eliminated the problems of gear misalignment and stress in the gearbox ofthe PW8000 GTF. It seems to be more likely that this has been achieved through theirpatented squirrel-cage bearing (Kostka, 2010), but based on the high temperature toleranceof AMBs, a magnetic bearing in a gas turbine engine does not appear to be too far off.The use of magnetic bearings for gas turbine engines has been studied in depth, and paperson the topic point out a number of their potential benefits, as well as their shortcomings.Benefits of magnetic bearings include durability and damage tolerance (Clark et al., 2004),

much smaller frictional losses (Schweitzer, 2002), and increased reliability at a reducedweight. Magnetic bearings also offer the potential to eliminate lubricating oil systems andavoid bearing wear, and have already demonstrated their successful application in machinespindles, mid-sized turbomachinery, and large centrifugal compressors (Becker, 2010).Eliminating the oil system in a wind tunnel gearbox provides a very large potential benefit,as numerous wind turbine fires have been attributed to the oil in an overheated gearboxcatching fire. Figure 8 is a photograph of one of many wind turbines whose overheatedgearboxes caused the lubricating oil to catch fire.

Fig. 8. A utility scale wind turbine on fire (Photo: flickr).


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Rolling element bearings, currently used in wind turbines, are hindered by their relativelyshort lifetime when subjected to high loads. Both foil and magnetic bearings offer longerlifetimes, with magnetic bearings outperforming foil bearings when used in large rotatingmachinery under high loads and a relatively low speed (Clark, 2004). Large, heavily loaded,

and relatively slow rotating provides a nearly perfect description of a modern utility scalewind turbine generator.A common criticism of magnetic bearings is the high power requirement to generate amplecurrent to generate a magnetic field great enough to yield an ample magnetic force to handlethe large loads. This criticism is simply outdated, as recent advances in permanent magnetsallow similarly strong magnetic fields to be generated by said magnets instead of via acurrent. It is these same permanent magnet advances that have allowed the construction ofthe aforementioned direct-drive generators.Magnetic bearings appear well-poised to mitigate some of the current gearbox problems,but their application to wind turbines lies well behind the current state of development ofdirect-drive and torque splitting solutions. This solution has the potential to aid in the

solution of gearbox problems on the lower end of utility scale wind turbines, as it may beadaptable to existing gearbox designs with minimal design changes required. As thetechnology matures, magnetic bearings have the potential to allow conventional gearboxdesigns to approach turbine rated powers of as much as 4 MW, if specific design constraintscall for the use of a conventional gearbox.

6. Continuously Variable Transmissions, CVTs

Another option for solving the gearbox problem is the use of a Continuously VariableTransmission (CVT). This gearing design has only recently reached mass production inpassenger vehicles, although it has been in use for a long time on farm machinery, drillpresses, snowmobiles, and garden tractors. Transmissions of the CVT type are capable ofvarying continuously through an infinite number of gearing ratios in contrast to the discretevarying between a set number of specific gear ratios of a standard gearbox.It is this gearing flexibility that allows the output shaft, connected to the generator in windturbine applications, to maintain a constant rate of rotation for varying input angularvelocities. The variability of wind speed and the corresponding variation in the rotor rpmcombined with the fixed phase and frequency requirements for electricity to be transmittedto the electrical grid make it seem that CVTs in concert with a proportional Position,Integral, Derivative (PID) controller have the potential to significantly increase the efficiencyand cost-effectiveness of wind turbines.

One disadvantage of CVTs is that their ability to handle torques is limited by the strength ofthe transmission medium and the friction between said medium and the source pulley.Through the use of state of the art lubricants, the chain-drive type of CVT has been able toadequately serve any amount of torque experienced on buses, heavy trucks, and earth-moving equipment. In fact, the Gear Chain Industrial B.V. Company of Japan appears tohave initiated work on a wind application for chain-driven CVTs.In addition to being able to handle minor shaft misalignments without being damaged,CVTs offer two additional potential benefits to wind turbines. As reported by Mangliardiand Mantriota (1996, 1994), a CVT-equipped wind turbine is able to operate at a more idealtip speed ratio in a variable speed wind environment by following the large fluctuations inthe wind speed. When simulated in a steady wind stream, a power increase with the


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addition of a CVT was observed for wind speeds above 11 m/s, and at 17 m/s, the CVT-equipped turbine power was double that of a conventional configuration, while exhibitingonly a 20 percent increase in torque. These results suggest that the typical cut-out windspeed of 25 m/s, set to limit the shaft stress and other stresses, may possibly be reevaluated,

to reflect the lower shaft stresses and higher rotor efficiencies at higher wind speeds(Mangliardi & Mantriota, 1994). The dynamic results were even more promising, as a CVT-equipped turbine subjected to a turbulent wind condition demonstrated increasedefficiencies of on average 10 percent relative to the steady wind stream CVT example.Additionally, the CVT-equipped turbine simulation produced higher quality electricalenergy, as the inertia of the rotor helped to significantly reduce the surges that are ever-present in constant-speed wind turbines subjected to rapid changes in wind speed(Mangliardi & Mantriota, 1996). Mangliardi and Mantriota go on to determine theextraction efficiency of a CVT-equipped and a CVT-less wind turbine as a function of windspeed, and this is presented below in Fig. 9.

Fig. 9. Extraction efficiency of standard and CVT-equipped wind turbines as a function ofwind speed in a turbulent wind field (Mangliardi and Mantriota, 1996).

As observable in Fig. 9, a CVT-equipped wind turbine is more efficient than a conventionalwind turbine at extracting the energy of the wind over all but a narrow range of windspeeds. The wind speed range where the CVT-equipped turbine is at a disadvantage iscentered on the design point of the conventional wind turbine, where both turbines exhibitsimilar aerodynamic efficiencies, but the CVT-equipped turbine is hampered by energylosses in its gearing system. It should be noted that this is a rather narrow range, and thevalue by which the CVT wind turbine trails the conventional wind turbine is much smallerwhen compared to its benefits over the rest of the range of wind speeds.


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As one moves from an ideal constant and uniform wind field to a turbulent wind field, thepotential benefits of a CVT-equipped wind turbine increase. The ratio of efficiency of a CVTwind turbine to a conventional one, R, increases (Mangliardi and Mantriota, 1996).Potential challenges to turbines equipped with a CVT center mainly on the lack of

knowledge about the scalability of such designs. Questions such as what is the upper limitto the amount of torque that may be transmitted through a belt drive have yet to beanswered. The potential benefits exist, but it appears that more research and turbine test

platforms are needed before the range of applicability of CVTs on wind turbines is known

(Department of Energy, 2010) and their commercial benefits quantified. Hydrostatic drives

are one type of CVT that has been studied for wind turbine applications, but it appears, atleast initially, that this may replace one problem, gearbox oil filtration, with another,

increased maintenance and hydraulic fluid cleanliness requirements.

7. Discussion

According to Fig. 10, gearbox failures account to 5 percent of wind turbine failures.However, they are costly compared with the other failures when they occur.

Fig. 10. Percentage of needed repairs and maintenance on utility scale wind turbines. Data:AWEA.

27

16

11

9

76 6

5 54

2 2

0

5

10

15

20

25

30

Electricalsystems

Electroniccontrol

unit

Hydraulicsystem

Sen

sors

Yawsystem

Rotor

hub

Rotorblades

Gear

box

Mechanicalb

reak

Structuralparts,Hou

sing

Drivet

rain

Generator


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While wind turbines are designed for a lifetime of around 20 years, existing gearboxes haveexhibited failures after about 5 years of operation. The costs associated with securing acrane large enough to replace the gearbox and the long downtimes associated with such arepair affect the operational profitability of wind turbines. A simple gearbox replacement

on a 1.5 MW wind turbine may cost the operator over $250,000 (Rensselar, 2010). Thereplacement of a gearbox accounts for about 10 percent of the construction and installationcost of the wind turbine, and will negatively affect the estimated income from a windturbine (Kaiser & Frhlingsdorf, 2007).Additionally, fires may be started by the oil in an overheated gearbox. The gusty nature ofthe wind is what degrades the gearbox, and this is unavoidable.Figure 11 summarizes the estimates of the economic rated power ranges of applicability foreach of the considered wind turbine gearbox solutions.The direct-drive approach to the current wind turbine gearbox reliability problem seems tobe taking a strong hold in the 3 MW and larger market segment, although torque splitting is

also being used in this range.For the 1.5 to 3 MW range however, multiple viable options exist or show potential,including torque splitting, magnetic bearings, and Continuously Variable Transmissions(CVTs). These options may gain traction over direct-drive solutions due to theapproximately 30 percent cost premium of a direct-drive system, and the larger sizes andcapital costs associated with such a system.If the magnetic bearing route is to be used, the answer may lie with gas turbine manufacturers,as their design criteria already call for bearings that are highly reliable, damage tolerant, andcapable of handling large loads. CVTs appear to also offer aerodynamic efficiency benefits

Fig. 11. Identified rated power applicability ranges of existing and possible wind turbinegearbox options. CVT: Continuously Variable Transmission.


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to wind turbines, but they may be limited by the amount of torque that may be transmittedby chain, belt, or hydrostatic means. For this reason, magnetic bearings appear to provide apotential solution to a slightly wider range of turbine rated powers than CVTs would.

8. References

Becker, K.H. (2010) Magnetic Bearings for Smart Aero Engines (MAGFLY). Proceedings of the13th International Symposium on Transport Phenomena and Dynamics of RotatingMachinery (ISROMAC-13), G4RD-CT-2001-00625, Honolulu, Hawaii, April 2010.

Burton, T., Sharpe, D, Jenkins, N, Bossany, E. (2004). Wind Energy Handbook (3rd Ed.). JohnWiley & Sons Ltd., ISBN: 0-471-48997-2, West Sussex, England.

Clark, D.J. Jansen, M.J., Montague, G.T. (2004). An Overview of Magnetic BearingTechnology for Gas Turbine Engines. National Aeronautics and Space Administration,NASA/TM-2004-213177.

Department of Energy (2010). Advanced Wind Turbine Drivetrain Concepts: Workshop

Report. Key Findings from the Advanced Drivetrain Workshop, Broomfield, Colorado,June 2010.

Enercon (2010). Enercon Wind Energy Converters: Technology & Service. Available from:

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